1. Field of the Invention
The invention relates to sound transducer devices, and more particularly to microphones with dynamically controllable operating characteristics. The invention further relates to a method of making and using such devices.
2. Related Art
Sound is propagated through a medium such as air by means of wave motion. Sound pressure level is the incremental variation from the static pressure in the air absent a sound wave. Each sound corresponds to a unique frequency which is the number of times a sound pressure varies from the static pressure level during a given time period. The cycle typically used is the Hertz (Hz) in which 1 Hz is equal to 1 cycle per second. A cycle is thus defined as a variation from equilibrium, a return to equilibrium, a negative variation from equilibrium, and a return to equilibrium. A sound travels through air as a wave at its corresponding frequency and sound pressure level.
A microphone is a sound wave transducer. A microphone typically includes a surface called a diaphragm which vibrates in response to sound waves incident thereon. The diaphragm is coupled to circuitry which translates the diaphragm vibrations into electrical signals which are proportional to the sound waves. In an electrostatic microphone the electrical signals are generated by detecting a variation of capacitance between the vibrating diaphragm and a fixed surface. For example, in a dc-biased electrostatic microphone, a capacitor is formed by two electrically conductive surfaces (a fixed backplate and the diaphragm) having an air gap between them and a voltage applied across them. An electret-biased electrostatic microphone is another example. This type of microphone also utilizes two surfaces to form a capacitor, but a permanently charged dielectric, such as Teflon.RTM., is attached to the one of the surfaces. In one example, the dielectric is cemented onto the backplate and forms a capacitance with the diaphragm. In another example, the dielectric is metalized on one side and used as the diaphragm to form a capacitance with the backplate.
In both the dc-biased and electret-biased electrostatic microphones, sound waves produce a vibration of the diaphragm. Circuitry connected to the diaphragm then generates an output electrical signal corresponding to the variation of the capacitance. These signals are typically then further amplified and processed as required. Such microphones are used in numerous devices, such as telephones, tape recorders, and intercoms. More recently, computer-based devices have utilized microphones for speech recognition, tele-conferencing, and in multi-media systems.
The diaphragm of a conventional microphone has a fixed compliance defined by its mass, the stiffness of its material, and by the restoring forces applied to the diaphragm. The restoring forces include the resiliency of the diaphragm material, the mechanical tension on the diaphragm, and the acoustic stiffness of the air gap. In some microphones, the backplate is perforated with holes to decrease the acoustic stiffness of the air gap. As used herein, more compliant means more flexible and less compliant means less flexible.
To a large extent, the operating characteristics of a microphone depend upon the compliance of the diaphragm. For example, if sound waves in the vicinity of the microphone are not of sufficient sound pressure level to move the diaphragm against the restoring forces, the diaphragm will not move. Similarly, if the sound wave is not of sufficient frequency to overcome the restoring forces on the diaphragm, the diaphragm will not vibrate. On the other hand, if sound waves in the vicinity of the microphone have a sound pressure level that greatly exceeds the restoring forces on the diaphragm, the diaphragm will flex in response to the sound pressure but the position of the diaphragm will not accurately correspond to the instantaneous sound waves, and clipping distortion will occur. Similarly, if the sound wave has a frequency in excess of the diaphragm's ability to flex and return, the frequency of the diaphragm's vibration will not correspond to the frequency of the sound wave.
Two characteristics of a microphone which are based on the compliance of the diaphragm are its "dynamic range" and its "frequency response." The dynamic range is defined by the difference between the microphone's minimum sound pressure level (SPL) (the most quiet sounds detectable by the microphone), and its maximum SPL (the loudest sounds the microphone can convert to electrical signals without distortion). The frequency response is defined by the range of sounds the microphone can detect. For a typical microphone, these are within the spectrum of human hearing. For example, a silicon micromachined microphone such as the one provided by Noise Cancellation Technologies, Inc. has a dynamic range of 160 decibels (dB) (based on a minimum SPL -40 dB and a maximum SPL of 120 dB) and a frequency response of 20 Hz (the low end of the human hearing spectrum) to 10,000 Hz (the high end of human speech).
Within this general spectrum of characteristics, special purpose microphones are commercially available with specific operating characteristics optimized for use in various applications. However, a microphone which is optimized for one application may not be suitable for another application. For example, a spectrum of typical sound pressure levels measured in dBs can include a quiet office, with a SPL of approximately 30 dB; ordinary human conversation, with an SPL of approximately 40-50 dB; and factory machinery, with an SPL of approximately 80 dB. Therefore, a microphone with a maximum SPL of 60 dB will be a good microphone for a speakerphone in a quiet office, but will be a poor microphone for an intercom on a factory floor. On the other hand, a microphone with a minimum SPL of 60 dB and a maximum SPL of 120 dB will be a good microphone for an intercom on a factory floor, but will be a poor microphone for use in a speakerphone in a quiet office.
Many microphones may be subject to a variety of conditions. A conventional microphone with fixed operating characteristics may not operate effectively across the entire range of conditions for a particular application. For example, an environment such as a factory floor has generally noisy conditions, but may also be occasionally quiet such as during breaks or after working hours. Thus, an intercom might be optimized to communicate between the factory floor and other areas of the factory when the factory is noisy. Such an intercom, which typically comprises a microphone, a speaker, and associated circuitry to facilitate communication with other intercoms, may operate poorly when the factory is quiet, due to its fixed operating characteristics.
In operation, a person wishing to communicate via the intercom speaks in the vicinity of the microphone. The microphone operating as a conventional microphone as described above, produces an electrical signal which is transmitted to another intercom where it is amplified to drive a speaker and be heard by a listener.
When the factory floor is noisy, a person trying to communicate therefrom must shout to be heard above the noise. Because the person speaking must shout, an intercom optimized for the factory floor will have a microphone with a relatively high maximum SPL. As discussed above, to achieve a high maximum SPL the compliance of the diaphragm must be relatively stiff. This is satisfactory for when the factory floor is noisy, however, a stiffer compliance also results in a higher minimum SPL. The higher minimum SPL, which is the lowest sound pressure level which will cause the diaphragm to vibrate, requires a greater sound pressure level to vibrate the diaphragm.
With a microphone as described above, when the factory floor is quiet, a person would still have to speak loudly to effectively use the intercom which was designed to operate optimally in a noisy factory environment. Therefore, such a microphone cannot operate optimally across the range of operating conditions. Furthermore, other areas of the factory which communicate through the intercom system may include a relatively quiet office. Since the microphone for the office will optimally have a lower maximum and minimum SPL, the same type of intercom unit cannot be used optimally in both the factory floor and the office environments.
Another example of a variable environment is in the area of computer-based microphone applications. It is sometimes desirable, such as in a speech recognition application, to have the microphone optimized for the speech of a specific person. Alternatively, it is sometimes desirable to have the microphone respond to a broader range of sounds, such as in a multi-party tele-conferencing application. In order to have the microphone respond to the speech of a specific person, a highly directional microphone is used and the person should speak in close proximity to the microphone. An omni-directional microphone, on the other hand, cannot typically focus on one specific person without also picking up sounds from other directions, such as in the vicinity of the speaker.
When sound conditions in an environment vary, a microphone with fixed operating characteristics may be unable to operate effectively in certain variations of the environment. A microphone with dynamically controllable operating characteristics is therefore needed to provide the flexibility to optimize the microphone to operate effectively in each variation of the environment.